Seam welding just like spot welding depends upon the proper application of pressure current and time. Let’s assume that the current and time are controlled properly. If the weld force varies during the welding process to lower values, the process is not stable. As the force decreases the contact between the seam weld wheel and the workpiece is not as good as it was with full force. The contact resistance (Rc) between the weld wheel and the workpiece will increase. Rc will become very large and a lot of heat will be generated on the part surface against the seam weld wheel face. This is more heat than the process was designed for with the proper force. Therefore the wheel surface will be over heated and will wear at an increased rate. Mushrooming will increase and if the wheel is being dressed larger amounts of material will be removed. The wheel will be dressed away at a faster rate and need replacement.
The seam weld wheel will fail prematurely due to low or inconsistent force application.
Seam welds are merely a line of spot welds. If their individual welds are weak it is caused by the same factors that cause spot welds to be weak. Some factors could be:
The size of the weld nugget
Indentation which indicates the wheel forging action will influence strength
Presence of cracks or inclusions in the weld nugget
Penetration of the nugget into both materials being welded
Brittleness of the material being welded. Tempering may be needed
The material being welded. Aluminum tends to form voids that need to be forged closed.
Proper wheel dressing
Burrs in seam welding might be describing weld flash or material that escaped from under the weld wheel during the weld and forms an irregular ridge along the length of the seam weld. All resistance welds seam or spot can leave some deformed material around the periphery of the weld zone. Generally this is not enough to be called a burr but in some cases the indentation would be cosmetically offensive if the surface were to be subsequently painted or were in a very visible location like the hood of a car. Product designers avoid welds in these locations.
Where there must be welds and indentations or slight raised areas are present, they can be treated as cosmetic welds. The weld schedule and electrodes, in this case weld wheels are selected to reduce the amount of heating and expulsion on the side of the part where the burr condition is not wanted. The wheel weld face might be increased. The force may be increased. This reduces contact resistance and surface heating. Adjustments to current and weld speed can be made. All of these would be made with the intent of reducing the surface heat and amount of material being expelled or pushed out during the welding process.
Yes, seam welding wheels can be dressed on the machine. On some machines the wheels are driven by knurled drive wheels that drive and also maintain the shape of the wheel at the same time. The photo below shows a knurl drive system. The wheels are continually driven and dressed each revolution.
Seam welding drive shafts carry current, deliver force and rotate during the seam welding operation. Duty cycles, currents and forces are high. The shaft operates in a housing which is filled with conductive grease and has current carrying shoes which ride on the shaft to deliver current. Eventually these conductors and the shaft wear and service is required. Sometimes people ask if they can the buildup the worn shaft with new copper and machined it back to size? They are suggesting the use of plasma or some welding process to lay down this layer of copper. Unfortunately the resultant laid on copper has some amount of porosity when compared to the original copper it is replacing. This soft porous copper will not wear well or conduct the high currents experienced in seam welding.